(1) Field of the Invention
The present invention relates to an electronically controlled automatic transmission for vehicles such as, for example, vehicles having a multi-stage automatic transmission, dump cars, trailer tractors motor graders, automobiles and the like.
Further, the present invention relates to an electronically controlled automatic transmission for vehicles in which an electronic control unit controls supply and discharge of pressurized oil on the basis of signals from a shift lever and running state detecting means to attain a desired speed change or transmission stage.
(2) Description of the Prior Art
An example of a conventional transmission for vehicles is schematically illustrated in FIG. 23. The transmission includes a multi-stage multi-plate clutch and gear. Thus, an operator moves a shift lever to a speed-stage in accordance with a speed manually to switch a hydraulic valve through a rod, wire or the like and selects a speed change or transmission stage to make operation running.
Another example of a conventional automatic transmission for vehicles is schematically illustrated in FIG. 24. In the transmission, an operator moves a shift lever into a unidirectional shift path to attain automatic speed change operation or transmission operation.
The conventional automatic transmission for vehicles illustrated in FIG. 24 has problems as follows:
(1) The conventional automatic transmission of FIG. 24 includes the shift lever moved in only one direction. In this shift lever, when a transmission having a number of speed change stages or transmission stages is automatically changed, for example when automatic speed change operation or transmission operation for eight stages is attained, it is necessary to provide many positions such as P (parking), N (neutral), D (automatic speed change OF automatic transmission), 8, 7, 6, 5, 4, 3, 2 and 1 in order to be able to select all speed stages manually by the operator and automatically. Accordingly, there is a problem that large space is occupied practically. In addition, operation is difficult and operation error tends to occur.
(2) In an automobile having an automatic transmission, normally, manual speed change operation or manual transmission operation is impossible at a high stage, and the automobile is used with high engine rotation during high-speed operation at D (automatic transmission) position in order to produce drive force. Accordingly, when a running speed is increased due to variation of a running resistance or slope, a shift-up operation is made unintentionally and lack of drive force occurs disadvantageously. Accordingly, manual transmission at a high-speed stage is desirable.
(3) When a multi-stage transmission operation, for example 8-stage transmission operation is attained in a vehicle having only manual transmission, 8 or more transmission positions are required. Accordingly, there is a problem that shift error by the operator occurs and transmission operation is troublesome.
A transmission mounted in a construction and civil engineering machinery such as a motor grader, a self-running scraper (motor scraper), a bulldozer and a wheel-type tractor shovel (wheel loader) or an industrial vehicle such as a forklift truck and a straddle carrier is required to cover a speed change area or transmission area from a very low speed area for original work to a normal running speed for running on a general road. Accordingly, as shown in FIG. 23, 6 to 8 transmission stages for both of forward and backward running are provided in general.
As described above, the transmission mounted in the construction and civil engineering machinery and the industrial vehicle includes very many transmission stages. Accordingly, in order to make small its structure and facilitate its transmission operation, the transmission adopts a direct power shift transmission (hereinafter referred to as a DSP) disclosed in Japanese Provisional Publication (Kokai) No. 62-255621 (1987). The DSP includes friction engagement elements such as clutch and brake assembled to rotating elements constituting a plurality of planetary gears in which an input axial end of the rotating elements is directly coupled with an output axis of an engine without a fluid coupling such as a torque converter. Thus, the DSP electrically controls selective supply and discharge of pressurized oil with respect to the friction engagement elements to thereby couple any rotating element of the planetary gears to the input axis of the transmission or fix the rotating element to a casing of the transmission, so that a transmission ratio is switched in accordance with a position of the transmission stage of the shift lever selected on the basis of operation of a driver.
The construction and civil engineering machinery and the industrial vehicle are to make special work and accordingly very many operation levers and switches are disposed around a driver's seat in addition to a steering handle and a shift lever. For example, a motor grader includes about ten operation levers disposed in a steering console provided with a steering handle to operate a blade and a scarifier. Further, a shift lever, many switches for lighting, various meters are disposed adjacent to the driver's seat.
Accordingly, operation of the operation levers, switches and the shift lever are very troublesome. Smooth operation of the construction and civil engineering machinery and the industrial vehicle requires very high skill.
(4) However, when various circumstances such as a great age of skilled operators and reduction of the absolute number of skilled operators are considered, it is desirable that the operability is made as simple as possible and the construction and civil engineering machinery and the industrial vehicle (hereinafter referred as vehicles) can be operated relatively easily by even other person than the skilled operator.